Heat stabilized sub-stoichiometric dielectrics

ABSTRACT

A sub-stoichiometric oxide, nitride or oxynitride layer in an optical stack, alone or in direct contact with one or two stabilizing layers, stabilizes the optical properties of the stack. The stabilizing layer(s) can stabilize the chemistry and optical properties of the sub-stoichiometric layer during heating. The change in optical characteristics of the sub-stoichiometric layer upon heating can counter the change in optical characteristics of the rest of the optical stack.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sub-stoichiometric dielectricmaterials. In particular, the present invention relates tosub-stoichiometric dielectric material layers used in optical stacksformed on transparent substrates.

2. Discussion of the Background

Sub-stoichiometric dielectrics are well known in the field of thin filmsand optical coatings. These sub-stoichiometric materials are typicallysubstances, based on metals, silicon or germanium, that are less thanfully reacted with oxygen or nitrogen. Frequently, these materials areoptically absorbing in the visible wavelengths while the fully reactedcorresponding compounds are often optically non-absorbing in the visiblewavelengths.

Sub-stoichiometric materials in thin film form are often included intooptical stacks. Compared to stoichiometric dielectric compounds,sub-stoichiometric dielectric materials have a number of desirableproperties.

For example, a sub-stoichiometric dielectric generally has a higherindex of refraction (“n”) than the corresponding stoichiometricdielectric, and may provide an optical stack with optical propertiesthat are more difficult to reach with all lower index stoichiometriclayers. The higher index of the sub-stoichiometric dielectric oftenallows the sub-stoichiometric layer to be thinner than the correspondingstoichiometric dielectric.

A sub-stoichiometric layer also generally has a higher extinctioncoefficient (“k”) than the corresponding stoichiometric dielectric. As aresult, sub-stoichiometric dielectric materials allow optical stacks toachieve optical properties that cannot be reached with non-absorbingstoichiometric dielectric compounds only.

Thin films of metal sub-oxides and sub-nitrides generally have betterproperties as chemical barriers than the corresponding stoichiometricmetal oxides and nitrides. Barrier layers of metal sub-oxides andsub-nitrides are useful in optical stacks to protect vulnerable metallayers from corrosion. The barrier layers reduce diffusion into thestack of reactive materials such as water or oxygen.

Deposition rates are usually higher for sub-stoichiometric oxide andnitride materials than for stoichiometric materials. As a result, theuse of sub-stoichiometric layers typically decreases production costsfor manufacturing thin film coatings. This is true for most sputteringand evaporation processes.

For some optical designs, it is desirable to block transmission of UVlight. Most sub-stoichiometric materials tend to be more absorbing inthe UV wavelengths than the corresponding stoichiometric compounds.

An undesirable property of sub-stoichiometric thin film materials isthat they tend to be more chemically reactive than fully oxidized orfully nitrided compounds. Often, a sub-stoichiometric layer in anoptical stack will oxidize, particularly if the stack is heated orsubjected to water or corrosive chemicals. Oxidation can result in achange in the layer's n and k values, which will change the spectralcharacteristics of an optical stack.

There is a need to stabilize the properties of optical stacks,particularly optical stacks containing sub-stoichiometric layers. Thisis particularly true when the stack is heated in an annealing orsubstrate tempering process.

SUMMARY OF THE INVENTION

The present invention provides an optical stack containing asub-stoichiometric dielectric layer whose characteristics are controlledto stabilize the optical properties (e.g., transmission and reflection)of the stack.

In embodiments, the sub-stoichiometric layer can be directly contactedon one or both sides by a stabilizing or cladding layer that functionsto stabilize the chemistry of the sub-stoichiometric layer. Thestabilization function occurs, e.g., when the optical stack is exposedto elevated temperatures that would normally cause thesub-stoichiometric dielectric layer to react in air or anotheratmosphere. By hindering reaction of the sub-stoichiometric layer andstabilizing the chemistry of the optical stack, the stabilizing layer(s)can stabilize the optical properties of the stack, particularly when thesub-stoichiometric layer serves a primarily optical interferencefunction in the stack.

In other embodiments, the composition and thickness of asub-stoichiometric layer can be chosen so that reaction of thesub-stoichiometric layer with air or another atmosphere upon heatingbrings about a change in an optical property (e.g., optical absorption)that exactly balances the change in that property by the rest of theoptical stack, so that the stack exhibits zero net change in the opticalproperty.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of this invention will be described in detailwith reference to the following figures.

FIG. 1 shows index of refraction (“n”) and extinction coefficient (“k”)values for silicon aluminum nitride sputtered from a target of siliconcontaining 10 weight % aluminum in nitrogen sputtering gas.

FIG. 2 shows an optical stack including a sub-stoichiometric dielectriclayer in direct contact with a stoichiometric dielectric stabilizinglayer.

FIG. 3 shows an optical stack including a sub-stoichiometric dielectriclayer sandwiched between and in direct contact with two stoichiometricdielectric stabilizing layers.

FIG. 4 shows index of refraction, n, and extinction coefficient, k, fora 39 nm thick NiCrO_(x) sample before and after heating at 730° C. forfour minutes in air.

FIG. 5 shows the change in transmission (Delta % TY) upon tempering of asingle silver, low-emissivity optical stack containing a NiCrOx layer asa function of the NiCrOx layer thickness.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides sub-stoichiometric layers, alone or incombination with one or more stabilizing layers, that can stabilize theoptical properties of an optical stack, particularly during heating andtempering.

Due to process limitations, multi-element compounds are rarely depositedin thin film layers with the exact ratio of elements dictated bystoichiometry.

In light of this, the term “stoichiometric” as used herein refers to anoxide, nitride or oxynitride, of one or more elements, in which theatomic ratio of oxygen and/or nitrogen relative to the other elements iswithin ±5% of the atomic ratio of oxygen and/or nitrogen in an oxide,nitride or oxynitride compound of the one or more elements. For example,“stoichiometric” tin oxide refers herein to SnO_(x), where 1.9≦x≦2.1.“Stoichiometric” silicon nitride refers herein to Si₃N_(y), where3.8≦y≦4.2. A “stoichiometric” oxynitride of Si-10 wt % Al refers hereinto (Si_(0.9)Al_(0.1))₃(O_(x)N_(y))_(w), where x+y=1; as x approaches 1,w approaches 6; and as y approaches 1, w approaches 4.

The term “sub-stoichiometric” as used herein refers to an oxide, nitrideor oxynitride, of one or more elements, in which the atomic ratio ofoxygen and/or nitrogen relative to the other elements is less than 95%and at least 30% of the atomic ratio of oxygen and/or nitrogen in theoxide, nitride or oxynitride compound of the one or more elements. Forexample, a “sub-stoichiometric” tin oxide refers herein to SnO_(x),where 0.6≦x<1.9. “Sub-stoichiometric” silicon nitride refers herein toSi₃N_(y), where 1.2≦y<3.8.

The atomic ratio of oxygen and/or nitrogen in a stoichiometric orsub-stoichiometric layer can be determined using various techniques wellknown in the art. For example, measurements of n and k can be used tomake an estimation of stoichiometry. Values of n and k for manystoichiometric oxides and nitrides are well documented in the literatureand easily verified. n and k values for some standard samples ofspecific sub-stoichiometric materials can also be easily obtained.Comparison of these values with those of a particular stoichiometric orsub-stoichiometric layer can indicate the atomic ratio of oxygen and/ornitrogen in the particular layer. Surface analysis techniques such asx-ray photoelectron spectroscopy (XPS) and Rutherford backscattering(RBS) can also be used to determine stoichiometry.

The term “dielectric” as used herein refers to an oxide, nitride oroxynitride material, which can be “stoichiometric” or“sub-stoichiometric”, that is at least partially transparent to visiblelight and creates interference effects in thin films.

The term “homogeneous” as used herein describes a layer that does nothave a gradient in chemical composition extending from one surface ofthe layer to the other surface of the layer. Thus, the term“homogeneous” describes a layer consisting of a single compound. Theterm “homogeneous” also describes a layer consisting of a uniformmixture of two or more different compounds.

In embodiments, the present invention provides an optical stack having asub-stoichiometric dielectric layer in contact with one or twostabilizing layers. Preferably the stabilizing layers are in directcontact with the sub-stoichiometric layer.

The sub-stoichiometric layer is a sub-stoichiometric dielectriccomposition that can result from the reaction of oxygen and/or nitrogenwith at least one metal element or semiconductor element. Suitable metalelements include transition metals, Mg, Zn, Al, In, Sn, Sb and Bi.Preferably, the metal elements include Mg, Y, Ti, Zr, Nb, Ta, W, Zn, Al,In, Sn, Sb and Bi. Suitable semiconductor elements include Si and Ge.The sub-stoichiometric layer can be doped with oxides, nitrides andoxynitrides of elements such as Ti, Fe and Cu that raise the n and k ofthe layer. Co-sputtering the dopants and the host material may produce alayer having a non-homogeneous composition, e.g, a layer in which thedopant concentration varies from top to bottom. Preferably, thesub-stoichiometric layer has a homogeneous composition.

The sub-stoichiometric layer preferably functions in an optical stackprimarily as an optical interference layer. The sub-stoichiometric layercan have a thickness in a range of from 10 to 100 nm, preferably from 15to 80 nm, more preferably from 25 to 70 nm. If the sub-stoichiometriclayer is less than 10 nm thick, then it may not sufficiently influencethe optical interference. If the sub-stoichiometric layer is more than100 nm thick, then it may absorb too much visible light and darken theoptical stack.

As discussed above, the sub-stoichiometric dielectric layer can be indirect contact with one or two stabilizing layers. Materials that can beused in the stabilizing layer are those that reduce chemical and opticalchanges in the adjacent sub-stoichiometric layer upon exposure to heat.Preferably each stabilizing layer has a homogenous composition.

A stabilizing layer can be a metallic material that would tend tooxidize to a substantially transparent compound during a heating processin an oxygen-containing atmosphere. A metal stabilizing layer cancomprise, e.g., Ti, Zr, Hf, Nb, Ta, Mo, W, Al, or Mg, and alloys,aluminides and silicides of these elements.

A stabilizing layer can also be a sub-stoichiometric dielectric. Whenthe stabilizing layer is a sub-stoichiometric dielectric, thestabilizing layer has a different composition than that of thesub-stoichiometric layer contacted by the stabilizing layer. Thesub-stoichiometric stabilizing layer upon heating may retain itssub-stoichiometric state or may oxidize to a more stoichiometric state.

Preferably, the stabilizing layer is a stoichiometric dielectric. Astoichiometric stabilizing layer can be a fully reacted version of thesub-stoichiometric layer contacted by the stabilizing layer, with themetal and/or semiconductor elements in the oxides, nitrides oroxynitrides of the stoichiometric layer being in their highest oxidationstate (e.g., Nb₂O₅, with Nb⁵⁺). Alternatively, the stabilizing layer canbe a stoichiometric version of the oxides, nitrides or oxynitrides ofthe sub-stoichiometric layer in which the elements are not in theirhighest oxidation state (e.g., NbO₂, with Nb⁴⁺). The stabilizing layercan also be a stoichiometric, partially or fully reacted, oxide, nitrideor oxynitride of different elements than those present in thesub-stoichiometric layer.

The stoichiometric stabilizing layer(s) can be more or less absorbing tovisible light than the sub-stoichiometric layer. Many elements, such asMg, Y, Ti, Zr, Nb, Ta, W, Zn, Al, In, Sn, Sb, Bi and Ge, formsub-stoichiometric oxides, nitrides and oxynitrides that are moreoptically absorbing to visible light than is the most fully reactedstoichiometric oxide, nitride or oxynitride of the element. In contrast,some metals, such as Cr, Fe, Ni and Cu, have more than onestoichiometric oxidation state, and in some cases the most oxidizedstate is not the least absorbing to visible light. Heating a thin filmof these oxides, and subsequently causing further oxidation, can resultin a more optically absorbing layer. For example, Cu₂O and CuO are thetwo stoichiometric oxides of copper, and Cu₂O is the least absorbing inthe visible wavelengths.

Oxides of Mg, Y, Ti, Zr, Nb, Ta, W, Zn, Al, In, Sn, Sb, Bi and Ge tendto have an extinction coefficient k of close to zero (essentiallynon-absorbing) throughout the visible wavelengths in the fully oxidizedstate. Nitrides of Al and Si follow the same trend. This non-absorbingcharacteristic makes stabilizing layers of these materials very usefulin optical stacks. For many optical designs, absorption is highlyundesirable. However, there are some optical designs where absorption isacceptable or desirable.

FIG. 1 compares index of refraction and extinction coefficient valuesfor sub-stoichiometric and stoichiometric SiAlO_(x)N_(y) sputteredfilms. FIG. 1 shows that sub-stoichiometric SiAlO_(x)N_(y) has a higherindex of refraction and higher extinction coefficient thanstoichiometric SiAlO_(x)N_(y).

The stabilizing layer functions primarily to chemically stabilize thesub-stoichiometric layer. Preferably, the stabilizing layer(s) incontact with the sub-stoichiometric layer allow the sub-stoichiometriclayer to be heated to a temperature of 600° C., more preferably 700° C.,even more preferably 800° C., for 4 to 5 minutes with little or nochange in optical properties.

A stabilizing, or cladding, layer can have a thickness in a range offrom 1 to 10 nm, preferably 2 to 8 nm, more preferably from 2 to 5 nm.

In embodiments, the index of refraction of the sub-stoichiometric layercan be at least 1.8 (i.e., n≧1.8), preferably at least 2.3 (i.e.,n≧2.3). The extinction coefficient of the sub-stoichiometric layer, k,can be in a range of 0.03≦k≦0.15.

The oxide, nitride or oxynitride of the sub-stoichiometric layerdielectric can have a higher index of refraction than the stabilizinglayer(s) and can be thicker than the one or more stabilizing layers.This can result in a combination of sub-stoichiometric andstoichiometric layers with a higher index of refraction than astoichiometric layer of the same thickness as the combination.

Alternatively, the oxide, nitride or oxynitride of thesub-stoichiometric layer dielectric can have a lower index of refractionthan the stabilizing layer(s). The combination with one or morestabilizing layers with such a low index sub-stoichiometric layer cancreate the equivalent of a layer having a much higher index ofrefraction than the sub-stoichiometric layer.

When the sub-stoichiometric layer is sandwiched between two stabilizinglayers, the two stabilizing layers can have the same or differentcompositions, and the same or different thicknesses.

The optical stack of the present invention can be made by conventionalthin film deposition techniques, such as sputtering, in which laminatesof various layers are formed by vapor depositing the layerssequentially.

Optical stacks of various metal and dielectric layers can reduce theemissivity of transparent substrates, such as glass. FIG. 2 illustratesa low-emissivity (“low-e”) optical stack 20 deposited on a glasssubstrate 21. The optical stack 20 includes metal layers 24 and 27,which can comprise an infrared radiation reflecting metal such as Ag, Cuor Au. Between glass substrate 21 and metal layer 24 issub-stoichiometric dielectric layer 22 in direct contact withstoichiometric dielectric layer 23. Between metal layers 24 and 27 issub-stoichiometric dielectric layer 25 in direct contact withstoichiometric dielectric layer 26.

FIG. 3 illustrates a low-e optical stack 30 deposited on a glasssubstrate 31. The optical stack 30 includes metal layers 35 and 39,which can comprise an infrared radiation reflecting metal such as Ag, Cuor Au. Between glass substrate 31 and metal layer 35 issub-stoichiometric dielectric layer 33 sandwiched between and in directcontact with stoichiometric dielectric layers 32 and 34. Between metallayers 35 and 39 is sub-stoichiometric dielectric layer 37 sandwichedbetween and in direct contact with stoichiometric dielectric layers 36and 38.

In FIGS. 2 and 3, the designation “ . . . ” indicates the presence ofone or more unspecified layers.

In other embodiments of the present invention, an optical stack isprovided in which the composition and thickness of thesub-stoichiometric layer is chosen so that the change in opticalproperties of the sub-stoichiochiometric layer upon heating offsets thechange in optical properties of the remainder of the stack. Upon heatingduring tempering, some sub-stoichiometric materials, such as sputteredsub-stoichiometric NiCrO_(x), become more absorbing to visible light andexhibit an increase in index of refraction. In contrast, most low-eoptical stacks become more transparent and lighten during tempering.Selection, by techniques well known in the art, of a suitable thicknessof a suitable sub-stoichiometric material that becomes more absorbing tovisible light upon heating will allow optical stacks to be designed thatwhen heated during tempering undergo an absolute change in transmission(change in % TY) of 1.00% or less, preferably 0.50% or less, morepreferably 0.25% or less (i.e., a change in transmission of ±1.00% orless, ±0.50% or less, or ±0.25% or less, respectively).Sub-stoichiometric NiCrO_(x) having a thickness of from 2 to 20 nm,preferably 3 to 12 nm, can be used to counter the increase intransmission on heating. If the NiCrO_(x) layer is too thin, then itwill be unable to counter the transmission changes in the other stacklayers. If NiCrO_(x) is too thick, it will overcome the transmissionincrease of the other stack layers and the optical stack will overallbecome less transparent and darken upon tempering. The thickness of theNiCrO_(x) can be designed to exactly balance the tendency of the otherlayers to lighten and create a zero tempering transmission change.Preferably the sub-stoichiometric NiCrO_(x) layer is homogeneous.

EXAMPLES

Examples of low-e coatings on glass utilizing the present invention areshown in the examples below. Dielectric coatings were formed bymidfrequency (˜30 kHz), dual magnetron sputtering onto room temperaturesubstrates. SiAlO_(x)N_(y) was sputtered from a SiAl target in anAr/O₂/N₂ atmosphere. ZnO was sputtered from a Zn target in an Ar/O₂atmosphere. (Si_(0.9)Al_(0.1))O_(x)N_(y) was sputtered from a targetcontaining Si and 10 wt % Al. Ag was DC sputtered from an Ag target inan Ar atmosphere. NiCrO_(x) was DC sputtered from a NiCr target in anAr/O₂ atmosphere.

The stoichiometry of oxides, nitrides and oxynitrides was controlled bycontrolling the compositions of the sputtering target and the sputteringatmosphere. Optical transparency was used to confirm the stoichiometryof stoichiometric nitrides containing silicon and stoichiometricoxynitrides containing silicon.

Example 1

A comparison was made of the change in optical properties upon temperingof a sub-stoichiometric SiAlO_(x)N_(y) layer not in contact with astabilizing stoichiometric SiAlO_(x)N_(y) layer relative to the samesub-stoichiometric SiAlO_(x)N_(y) layer when in direct contact with astabilizing stoichiometric SiAlO_(x)N_(y) layer.

In this comparison, low-e optical stacks containing a single Ag layerwere deposited on soda-lime glass substrates (5 cm×5 cm×0.3 cm). Tables1 and 2 show the numerical order in which various layers were deposited.Table 1 shows the deposition conditions used to form an optical stack inwhich the sub-stoichiometric SiAlO_(x)N_(y) layer was not in contactwith a stabilizing stoichiometric SiAlO_(x)N_(y) layer. Table 2 showsthe deposition conditions used to form another optical stack in whichthe sub-stoichiometric SiAlO_(x)N_(y) layer was in direct contact with astabilizing stoichiometric SiAlO_(x)N_(y) layer.

The size of each of the two sputtering targets used to depositSiAlO_(x)N_(y) by dual magnetron sputtering was 1 m×110 mm, for aneffective target surface area of 0.11 m². The SiAlO_(x)N_(y) wassputtered at a power of 17.8-18.0 kW, for a power density of about 16.3W/cm². Sub-stoichiometric SiAlO_(x)N_(y) was obtained by sputtering witha N₂ gas flow of 83 sccm. Stoichiometric SiAlO_(x)N_(y) was obtained bysputtering with a N₂ gas flow of 98 sccm.

TABLE 1 Gas Flow Ar O₂ N₂ AC Power No. Layer (sccm) (sccm) (sccm) (kW) 1SiAlO_(x)N_(y)(*) 100 10 83 17.8 2 ZnO 50 165  0 14.4 3 Ag 33 3 — — 4NiCrO_(x) 50 37 — — 5 SiAlO_(x)N_(y) 100 10 98 17.9 *Sub-stoichiometricSiAlO_(x)N_(y)

TABLE 2 Gas Flow Ar O₂ N₂ AC Power No. Layer (sccm) (sccm) (sccm) (kW) 1SiAlO_(x)N_(y)(**) 100 10 98 17.9 2 SiAlO_(x)N_(y) (*) 100 10 83 18   3ZnO 50 165  0 14.4 4 Ag 33 3 — — 5 NiCrO_(x) 50 37 — — 6 SiAlO_(x)N_(y)100 10 98 17.9 *Sub-stoichiometric SiAlO_(x)N_(y) **Stabilizingstoichiometric SiAlO_(x)N_(y)

The coated substrates were tempered in a muffle furnace at 730° C. for 4minute.

Table 3 shows the color change (ΔE) that resulted from the tempering. ΔE(Lab) is the color change including L, a* and b* values. ΔE(ab) is thecolor change including color only (a* and b*) but not intensity (L).

TABLE 3 Table 1 stack Table 2 stack (no (with stabilizing layer)stabilizing layer) ΔE(Lab) ΔE(ab) ΔE(Lab) ΔE(ab) T 2.0 1.5 2.3 1.5 Rg3.6 3.6 3.1 3.0 Rf 4.4 4.4 3.7 3.7 T = transmission Rg = glass(uncoated) side reflected color Rf = thin film coated side reflectedcolor

Table 3 shows that the transmission color change was not decreased bythe stabilizing stoichiometric SiAlO_(x)N_(y) layer. In contrast, Table3 shows that the reflected color change was decreased by the addition ofthe stabilizing stoichiometric SiAlO_(x)N_(y) layer in direct contactwith the sub-stoichiometric SiAlO_(x)N_(y) layer. In particular, thedecrease in glass side reflected color change (i.e., Rg ΔE) was 0.5 to0.6 color units. Table 3 shows that the properties of an optical stackincluding a sub-stoichiometric SiAlO_(x)N_(y) layer can be stabilized bydirectly cladding the sub-stoichiometric SiAlO_(x)N_(y) layer with astabilizing stoichiometric SiAlO_(x)N_(y) layer.

Example 2

Two single silver low-emissivity optical stacks were made. The firststack had a single sub-stoichiometric (Si_(0.9)Al_(0.1))O_(x)N_(y)bottom dielectric. The second stack had a thermally stabilizingstoichiometric (Si_(0.9)Al_(0.1))O_(x)N_(y) layer between thesub-stoichiometric (Si_(0.9)Al_(0.1))O_(x)N_(y) and the glass substrate.On top of each stack were identical (Si_(0.9)Al_(0.1))O_(x)N_(y) layers.The complete stack designs are shown below:

First Stack:

Glass/sub-stoichiometric(Si_(0.9)Al_(0.1))O_(x)N_(y)/ZnO/Ag/NiCrO_(x)/(Si_(0.9)Al_(0.1))O_(x)N_(y)

Second Stack:

Glass/stoichiometric (Si_(0.9)Al_(0.1))O_(x)N_(y)/sub-stoichiometric(Si_(0.9)Al_(0.1))O_(x)N_(y)/ZnO/Ag/NiCrO_(x)/(Si_(0.9)Al_(0.1))O_(x)N_(y)

Both samples were tempered and checked for color change from tempering.The color change expressed as ΔE a* b* was calculated by the followingequation:ΔE a*b*=(a* _(BB) −a* _(AB))²+(b* _(BB) −b* _(AB))²]^(0.5)where BB refers to color before tempering and AB refers to color aftertempering.

The tempering color change for transmission (T ΔE a* b*), glass sidereflection (Rg ΔE a* b*) and film (i.e., stack) side reflection (Rf ΔEa* b*) are shown in the following Table 4.

TABLE 4 First stack: without Second stack: with stoichiometricstoichiometric (Si_(0.9)Al_(0.1))O_(x)N_(y) (Si_(0.9)Al_(0.1))O_(x)N_(y)stabilization layer stabilization layer T ΔE a* b* 1.5 1.5 Rg ΔE a* b*3.6 3.0 Rf ΔE a* b* 4.4 3.7

Table 4 shows that the addition to the second stack of the protectivestabilizing layer of stoichiometric (Si_(0.9)Al_(0.1))O_(x)N_(y)resulted in a reduction in tempering color shift for both glass sidereflection and film (optical stack) side reflection.

In embodiments of the present invention, the stabilizing layer resultsin a tempering color shift for glass side reflection of 4.0 or less,preferably 3.5 or less, more preferably 3.0 or less, even morepreferably 2.5 or less. In addition, the stabilizing layer results in atempering color shift for film (optical stack) side reflection of 4.0 orless, preferably, 3.5 or less, more preferably 3.0 or less, even morepreferably 2.5 or less.

Example 3

The following optical stack was prepared:

6 mm glass substrate/20 nm stoichiometric SiAlO_(x)N_(y)/8 nm ZnO/12 nmAg/2 nm NiCrO_(x)/5 nm stoichiometric SiAlO_(x)N_(y)/55 nmsub-stoichiometric SiAlO_(x)N_(y)/5 nm stoichiometric SiAlO_(x)N_(y)/8nm ZnO/15 nm Ag/2 nm NiCrO_(x)/36 nm stoichiometric SiAlO_(x)N_(y)

The SiAlO_(x)N_(y) was sputtered from a target containing Si and 10 wt %Al.

In this optical stack the middle dielectric (i.e. stoichiometricSiAlO_(x)N_(y)/sub-stoichiometric SiAlO_(x)N_(y)/stoichiometricSiAlO_(x)N_(y)) is primarily a higher index version of SiAlO_(x)N_(y).This allows a lower physical thickness for the layer and a highersputtering rate in the deposition process.

The middle dielectric combination was deposited under the conditionsshown in Table 5.

TABLE 5 Gas Flow Ar O₂ N₂ AC Power (sccm) (sccm) (sccm) (kW)Stoichiometric 100 10 90 14.2 SiAlO_(x)N_(y) Sub-stoichiometric 100 1083.5 14.2 SiAlO_(x)N_(y)

The color of the optical stack is very sensitive to the index andoptical thickness of the middle layers. Normally, glass tempering at atemperature of 730° C. for 4 minutes would change the index and opticalthickness. However, this change is reduced by the stabilizingstoichiometric layers clad on both sides of the sub-stoichiometriclayer.

Example 4

Tin zinc oxide (SnZnO_(x)) is a common dielectric material used intemperable low-emissivity designs. For creating certain optical effectsin thin film designs, SnZnO_(x) can be deposited in a sub-stoichiometricstate.

An investigation was carried out to determine if optically absorbing,sub-stoichiometric SnZnO_(y)N_(x) could be thermally stabilized usingstoichiometric SiAlO_(y)N_(x) layers. Thin film structures were formedon quartz substrates as illustrated in the following Table 6.

TABLE 6 Layer 1 Layer 2 Layer 3, Sample 5 nm thick 25 nm thick 5 nmthick 1 SiAlO_(x)N_(y)** SnZnO_(x)N_(y)* SiAlO_(x)N_(y)** 2SiAlO_(x)N_(y)** SnZnO_(x)N_(y)* 3 SnZnO_(x)N_(y)* SiAlO_(x)N_(y)** 4SnZnO_(x)N_(y)* *Sub-stoichiometric **Stabilizing stoichiometric

The layers were sputtered from 1 meter long Twin-Mag targerts. Nitrogenwas added to the sub-stoichiometric SnZnO_(x)N_(y) to stabilize thesputtering process and reduce arcing. Run conditions for theSiAlO_(x)N_(y) and the SnZnO_(x)N_(y) are shown in the following Table7.

TABLE 7 Ar O₂ N₂ Line AC flow flow flow speed Thick power I V PressureLayer (sccm) (sccm) (sccm) (m/min) (nm) (kW) (amps) (volts) (mbar)SiAlO_(y)N_(x) 100 10 115 7.562 5 17.1 27.2 831 6.1E−3 SnZnO_(y)N_(x) 80176 60 3.267 25 11.9 14.6 471 5.7E−3

After the four samples were deposited, percent optical absorption wasmeasured in the visible wavelengths before and after baking. Baking wasdone at 670° C. for 5 minutes. The change in percent optical absorptionfrom the samples before bake to after bake at a wavelength of 400 nm isshown in Table 8 below.

TABLE 8 Change in percent absorption at 400 nm Layer Structurewavelength SnZnO_(x)N_(y)* −5.4 SiAlO_(x)N_(y)**/SnZnO_(x)N_(y)* −2.01SnZnO_(x)N_(y)*/SiAlO_(x)N_(y)** −4.18SiAlO_(x)N_(y)**/SnZnO_(x)N_(y)*/SiAlO_(x)N_(y)** −2.84*Sub-stoichiometric **Stabilizing stoichiometric

Table 8 shows that the absorption decrease due to heating was less forthe samples with a stoichiometric SiAlO_(x)N_(y) layer.

A similar investigation was made to determine if stoichiometricSnZnO_(x) layers would stabilize optically absorbing, sub-stoichiometricSnZnO_(x)N_(y). In these stacks, the decrease in absorption with heatingwas the same with and without the SnZnO_(x) layers.

Example 5

Double silver low emissivity stacks with two NiCrO_(x) layers were madeas shown below:Glass/SiAlO_(x)N_(y)/ZnO/Ag/NiCrO_(x)/SiAlO_(x)N_(y)/ZnO/Ag/NiCrO_(x)/SiAlO_(x)N_(y)

In one version of the stack, the NiCrO_(x) layers were approximately 2nm thick. In a second version of the stack, the NiCrO_(x) layers wereapproximately 4 nm thick. Optical transmission (TY) was measured beforeand after baking the stacks at 730° C. for four minutes. The results areshown in the following Table 9.

TABLE 9 Esti- mated TY TY Trans- NiCrO_(x) before after DC Ar O2 portthick- tem- tem- power flow flow speed ness pering pering (kW) (sccm)(sccm) (m/min) (nm) (%) (%) ΔTY 3.67 50 37 8 2 75.13 80.50 5.37 3.67 5037 4 4 75.25 77.27 2.02

Table 9 shows that the optical stack with the thicker NiCrO_(x) has asmaller transmission change (ΔTY) upon heating. The decrease in ΔTY withincrease in NiCrO_(x) thickness indicates that NiCrO_(x) decreases intransmission and becomes more optically absorbing upon heating.

Example 6

The index of refraction, n, and extinction coefficient, k, of a 39 nmthick NiCrO_(x) film deposited on a quartz subtrate were measured as afunction of wavelength using a Woollam M2000U Ellipsometer beforebaking. After baking at 730° C. for four minutes in air, the opticalconstants were measured again. As shown in FIG. 4, both n and k for theNiCrO_(x) were increased by the heating.

Example 7

Four single silver low-emissivity optical stacks were made in which onlythe thickness of the NiCrOx barrier layer, deposited on the Ag layer,was varied. The optical stacks had the following general design:

Glass/3.5 nm stoichiometric SiAlO_(x)N_(y)/17 nm sub-stoichiometricSiAlO_(x)N_(y)/6 nm ZnO/13.4 nm Ag/NiCrO_(x)/38 nm stoichiometricSiAlO_(x)N_(y)

The NiCrO_(x) was reactively DC sputtered from 1 meter long 80 wt %Ni-20 wt % Cr target. The SiAlO_(x)N_(y) was sputtered from a targetcontaining Si and 10 wt % Al. The sputtering conditions are shown in thefollowing Table 10.

TABLE 10 NiCrO_(x) Sputtering Conditions Sputtering Ar flow O₂ flow DCpower Current Target Pressure (sccm) (sccm) (kW) (amps) Voltage (mbar)50 37 2.23 3.6 624 1.18E−3

In the standard or control stacks, the NiCrO_(x) layers wereapproximately 2 nm thick. The thickness of the NiCrO_(x) layer wasincreased in the other stacks.

The stacks were tempered by heating in air at 670° C. for 6 minutes, 20seconds.

The transmission (% TY) was measured before and after heating. Theresults are shown in the following Table 11 and in FIG. 5.

TABLE 11 NiCrOx Thickness Before Temper After Temper Delta TY (nm) % TY% TY (change in % TY) 2.2 82.1 85.6 3.5 2.2 82.3 85.6 3.4 4.3 81.8 83.11.3 8.7 79.7 80.1 0.5 17.4 73.5 71.8 −1.7

Table 11 and FIG. 5 show that the change in % TY upon temperingdecreased with increasing NiCrO_(x) thickness and became negative abovea NiCrO_(x) thickness of about 10.5 nm. These results predict that aNiCrO_(x) thickness of about 10.5 nm in the optical stack will result inan optical stack that exhibits zero transmission change upon tempering.These results also indicate that zero transmission change upon temperingcan be achieved in optical stacks containing two or more silver layersby proper selection of NiCrO_(x).

Both before and after the tempering, the optical stacks were tested forwet brush durability using a standardized procedure involving brushingunder water. The brushing damage on each sample was visually quantifiedusing a standard scale. The results are show in Table 12 below, in which“% damage” refers to the surface area damaged by brushing.

TABLE 12 % damage % damage NiCrOx thickness caused by wet brushingcaused by wet brushing (nm) before tempering after tempering 2.2 0 402.2 0 40 4.3 0 10 8.7 0 2 17.4 0 0

Table 12 shows that before tempering the optical stacks were not damagedby the wet brushing. In contrast, after the tempering the stacks withthe thinnest NiCrO_(x) layers exhibited the most surface damage, but theamount of wet brush damage decreased with increasing NiCrO_(x)thickness.

The disclosure herein of a range of values is a disclosure of everynumerical value within that range. In addition, the disclosure herein ofa genus is a disclosure of every species within the genus (e.g., thedisclosure of the genus “transition metals” is a disclosure of everytransition metal species, such as Nb, Ta, etc.).

While the present invention has been described with respect to specificembodiments, it is not confined to the specific details set forth, butincludes various changes and modifications that may suggest themselvesto those skilled in the art, all falling within the scope of theinvention as defined by the following claims.

1. An optical stack comprising a sub-stoichiometric layer in directcontact with a stabilizing layer, wherein the sub-stoichiometric layerconsists of a homogeneous sub-stoichiometric composition selected fromthe group consisting of oxides, nitrides and oxynitrides, where thesub-stoichiometric composition comprises at least one element selectedfrom the group consisting of metal elements and semiconductor elements,and the sub-stoichiometric composition further comprises asub-stoichiometric amount of at least one element selected from thegroup consisting of oxygen and nitrogen; the stabilizing layer comprisesthe at least one element selected from the group consisting of metalelements and semiconductor elements, and a stoichiometric amount of theat least one element selected from the group consisting of oxygen andnitrogen; and the sub-stoichiometric layer is from 10 to 100 nm thickand thicker than the stabilizing layer.
 2. The optical stack accordingto claim 1, wherein the stabilizing layer is from 1 to 10 nm thick. 3.The optical stack according to claim 1, wherein the metal elements areselected from the group consisting of transition metals, Mg, Zn, Al, In,Sn, Sb and Bi; and the semiconductor elements are selected from thegroup consisting of Si and Ge.
 4. The optical stack according to claim1, wherein the metal elements are selected from the group consisting ofMg, Y, Ti, Zr, Nb, Ta, W, Zn, Al, In, Sn, Sb and Bi; and thesemiconductor elements are selected from the group consisting of Si andGe.
 5. The optical stack according to claim 1, wherein the at least oneelement selected from the group consisting of metal elements andsemiconductor elements comprises Si and Al.
 6. The optical stackaccording to claim 1, wherein the sub-stoichiometric compositioncomprises at least two members of the group consisting of oxides,nitrides and oxynitrides.
 7. The optical stack according to claim 1,wherein the index of refraction of the sub-stoichiometric layer ishigher than the index of refraction of the stabilizing layer.
 8. Theoptical stack according to claim 1, wherein the sub-stoichiometric layerhas an index of refraction, n, such that n≧2.3.
 9. The optical stackaccording to claim 1, wherein the sub-stoichiometric layer has anextinction coefficient, k, such that 0.03≦k≦0.15.
 10. The optical stackaccording to claim 1, wherein the optical stack is on a transparentsubstrate; the optical stack further comprises a metal layer; and thesub-stoichiometric layer and the stabilizing layer are between thetransparent substrate and the metal layer.
 11. The optical stackaccording to claim 10, where the transparent substrate comprises aglass.
 12. The optical stack according to claim 11, wherein the opticalstack is produced by a process comprising tempering the optical stack onthe transparent substrate; and the glass side reflection tempering colorshift of the optical stack on the transparent glass substrate is 3.0 orless.
 13. The optical stack according to claim 11, wherein the opticalstack is produced by a process comprising tempering the optical stack onthe transparent substrate; and the optical stack side reflectiontempering color shift of the optical stack on the transparent glasssubstrate is 3.7 or less.
 14. The optical stack according to claim 10,wherein the metal layer comprises Ag.
 15. The optical stack according toclaim 10, wherein the optical stack further comprises another metallayer; and the sub-stoichiometric layer and the stabilizing layer arebetween the metal layer and the other metal layer.
 16. The optical stackaccording to claim 1, wherein the optical stack is on a transparentsubstrate; the optical stack further comprises a metal layer; and thesub-stoichiometric layer and the stabilizing layer are outward from thesubstrate from the metal layer.
 17. A method of making an optical stack,the method comprising laminating a stabilizing layer and asub-stoichiometric layer; and producing the optical stack of claim 1.18. An optical stack comprising a sub-stoichiometric layer sandwichedbetween and in direct contact with a first stabilizing layer and asecond stabilizing layer, wherein the sub-stoichiometric layer consistsof a sub-stoichiometric composition selected from the group consistingof oxides, nitrides and oxynitrides, where the sub-stoichiometriccomposition comprises at least one element selected from the groupconsisting of metal elements and semiconductor elements, and thesub-stoichiometric composition further comprises a sub-stoichiometricamount of at least one element selected from the group consisting ofoxygen and nitrogen; and the first stabilizing layer and the secondstabilizing layer each comprises the at least one element selected fromthe group consisting of metal elements and semiconductor elements, and astoichiometric amount of the at least one element selected from thegroup consisting of oxygen and nitrogen; the sub-stoichiometric layer isfrom 10 to 100 nm thick and thicker than both the first and secondstabilizing layers.
 19. The optical stack according to claim 18, whereinthe first stabilizing layer and the second stabilizing layer have thesame composition.
 20. The optical stack according to claim 18, whereinthe first stabilizing layer and the second stabilizing layer havedifferent compositions.
 21. The optical stack according to claim 18,wherein the sub-stoichiometric layer is homogeneous.
 22. The opticalstack according to claim 18, wherein the sub-stoichiometric compositioncomprises at least two members of the group consisting of oxides,nitrides and oxynitrides.
 23. The optical stack according to claim 18,wherein the metal elements are selected from the group consisting oftransition metals, Mg, Zn, Al, In, Sn, Sb and Bi; and the semiconductorelements are selected from the group consisting of Si and Ge.
 24. Theoptical stack according to claim 18, wherein the metal elements areselected from the group consisting of Mg, Y, Ti, Zr, Nb, Ta, W, Zn, Al,In, Sn, Sb and Bi; and the semiconductor elements are selected from thegroup consisting of Si and Ge.
 25. The optical stack according to claim18, wherein the at least one element selected from the group consistingof metal elements and semiconductor elements comprises Si and Al. 26.The optical stack according to claim 18, wherein the index of refractionof the sub-stoichiometric layer is higher than the index of refractionof each of the first stabilizing layer and the second stabilizing layer.27. The optical stack according to claim 18, wherein thesub-stoichiometric layer has an index of refraction, n, such that n≧2.3.28. The optical stack according to claim 18, wherein thesub-stoichiometric layer has an extinction coefficient, k, such that0.03≦k≦0.15.
 29. The optical stack according to claim 18, wherein thefirst stabilizing layer is from 1 to 10 nm thick.
 30. The optical stackaccording to claim 29, wherein the second stabilizing layer is from 1 to10 nm thick.
 31. The optical stack according to claim 18, wherein theoptical stack is on a transparent substrate; the optical stack furthercomprises a metal layer; and the first stabilizing layer, thesub-stoichiometric layer, and the second stabilizing layer are betweenthe transparent substrate and the metal layer.
 32. The optical stackaccording to claim 31, wherein the transparent substrate comprises aglass.
 33. The optical stack according to claim 31, wherein the metallayer comprises Ag.
 34. The optical stack according to claim 31, whereinthe optical stack further comprises another metal layer; and the firststabilizing layer, the sub-stoichiometric layer, and the secondstabilizing layer are between the metal layer and the other metal layer.35. The optical stack according to claim 18, wherein the optical stackis on a transparent substrate; the optical stack further comprises ametal layer; and the first stabilizing layer, the sub-stoichiometriclayer, and the second stabilizing layer are outward from the substratefrom the metal layer.
 36. A method of making an optical stack, themethod comprising laminating a first stabilizing layer, asub-stoichiometric layer and a second stabilizing layer; and producingthe optical stack of claim 18.